I've attempted to make the body a little more flexible by altering the angle of the hips and chest object so that he's not so stiff when he leans forward — already, I think it's looking a great deal better. The arm makes a great deal more sense now that it follows the body. Initially, because it wasn't doing much, it was difficult to get the arm to do anything that didn't look ridiculously over-exaggerated.
It's still a little clunky, though, and we can easily see from the perspective view that the body's weight shifts quite unnaturally. I don't know if it's the head and arms getting in the way but he seems unbalanced somehow, and he seems to twist more in one direction than the other. The entire twisting seems a bit jerky. Probably need to muck around with some keyframes!